In a distributed power generation system, the inverter acts as a bridge connecting the renewable energy (such as photovoltaic power generation, wind power generation, etc.) with the power grid or load, and its performance directly affects the entire distributed power generation system. The multi-level inverter in the distributed power generation system has the following advantages compared with the two-level inverter: (1) the inverter output voltage and current harmonics are smaller; (2) the output voltage change rate is smaller; and (3) the output power is larger. Therefore, multi-level inverters have received attention and applications in distributed power generation systems.
In a multi-level inverter, the T-type three-level three-phase inverter has the following advantages compared with a diode-clamped three-level three-phase inverter: (1) reducing six power diodes, thereby reducing system cost; (2) having an inverter switching frequency in 5 kHz-30 kHz, therefore the T-type three-level three-phase inverter is more efficient than the diode-clamped three-level three-phase inverter. Therefore, T-type three-level inverters have been widely used in distributed power generation systems.
In photovoltaic power generation systems, inverters can be divided into isolated and non-isolated inverters, and non-isolated inverters are widely used in distributed photovoltaic power generation systems due to their advantages such as high efficiency and small size. For the non-isolated T-type three-level three-phase photovoltaic grid-connected inverters, in order to reduce the common-mode leakage current of photovoltaic grid-connected inverters, most commercial inverters usually adopt a scheme of connecting the common point of the output filter capacitor of the T-type three-level three-phase inverter to the neutral point (the circulation neutral line) of the DC bus capacitor, as shown in FIG. 1. In this way, a part of the high-frequency current of the inverter will be circulated in the neutral line, which greatly reduces the output common-mode leakage current of the inverter. However, due to the presence of the circulation neutral line of the inverter, the high-frequency component of resonance formed by the LC filter of the T-type three-level three-phase inverter will also flow in the inverter and the neutral line, affecting the currents in the inversion side and the circulation neutral line of the photovoltaic grid-connected inverter, causing the current of the output inversion side and the circulation neutral line of the photovoltaic grid-connected inverter current to contain the resonant current of the LC filter. Therefore, if uncontrolled, the high-frequency current of the circulation neutral line and the high-frequency component of the current of the inversion side will increase.
For most commercial photovoltaic grid-connected inverters, the resonant current of the LC filter is not considered. The main reason is that the resonant current of the LC filter mainly circulates on the inversion side and the circulation neutral line, and does not flow in the inverter grid side (will not flow into the grid). It can be seen that in order to suppress the high-frequency current of the circulation neutral line in the T-type three-level three-phase grid-connected inverter and the resonant high-frequency current of the inversion side of the inverter, a method for suppressing the current of the circulation neutral line in the T-type three-level three-phase photovoltaic grid-connected inverter to improve the performance of the T-type three-level three-phase photovoltaic grid-connected inverter has a good application prospect in the distributed power generation systems.